no-mutating-assign with any expression

[benadamstyles]

Thanks for this excellent plugin. Further to #17, I wonder if you'd consider allowing any function call as the first argument to Object.assign() (or at least add an option to allow this). For example, I think this would always be "functionally safe":

Object.assign(anyFunctionCall(_), {prop: 'value'})

If you're happy with the concept I'm happy to PR (although any hint you can give about what the eslint type of a function call would be, would be very helpful, as I've never written an eslint rule before)!

[graingert]

not safe:

const globalObj = {};
globalObj.prop; // undefined
const anyFunctionCall = () => globalObj;
const newObj = Object.assign(anyFunctionCall(_), {prop: 'value'});
newObj.value; // 'value'
globalObj.prop; // 'value'

[graingert]

safe:

const globalObj = {};
globalObj.prop; // undefined
const anyFunctionCall = () => globalObj;
const newObj = Object.assign({}, anyFunctionCall(_), {prop: 'value'});
newObj.value; // 'value'
globalObj.prop; // undefined

[benadamstyles]

Yep my mistake!